Enteric fevers and diarrheal diseases, e.g., typhoid fever and cholera, are major causes of morbidity and mortality throughout the developing world, Hook et al., 1980, In Harrison's Principles of Internal Medicine, 9th Ed., 641-848, McGraw Hill, New York. Traditional approaches to the development of vaccines for bacterial diseases include the parenteral injection of purified components or killed organisms. These parenterally administered vaccines require technologically advanced preparation, are relatively expensive, and are often, because of dislike for needle-based injections, resisted by patients. Live oral vaccine strains have several advantages over parenteral vaccines: low cost, ease of administration, and simple preparation.
The development of live vaccines has often been limited by a lack of understanding of the pathogenesis of the disease of interest on a molecular level. Candidate live vaccine strains require nonrevertable genetic alterations that affect the virulence of the organism, but not its induction of an immune response. Work defining the mechanisms of toxigenesis of Vibrio cholerae has made it possible to create live vaccine strains based on deletion of the toxin genes, Mekalanos et al., 1983, Nature 306:551, Levine et al., 1988, Infect. Immun. 56:161.
Recent studies have begun to define the molecular basis of Salmonella typhimurium macrophage survival and virulence, Miller et al., 1989, Proc. Natl. Acad. Sci. U.S.A. 86:5054, hereby incorporated by reference. Salmonella typhimurium strains with mutations in the positive regulatory regulon phoP are markedly attenuated in virulence for BALB/c mice. The phoP regulon is composed of two genes present in an operon, termed phoP and phoQ. The phoP and phoQ gene products are highly similar to other members of bacterial two-component transcriptional regulators that respond to environmental stimuli and control the expression of a large number of other genes. A mutation at one of these phoP regulatory region regulated genes, pagC, confers a virulence defect. Strains with pagC, phoP, or phoQ mutations afford partial protection to subsequent challenge by wild-type S. typhimurium.
Salmonella species cause a spectrum of clinical disease that includes enteric fevers and acute gastroenteritis, Hook et al., 1980, supra. Infections with Salmonella species are more common in immunosuppressed persons, Celum et al., 1987, J. Infect. Dis. 156:998. S. typhi, the bacterium that causes typhoid fever, can only infect man, Hook et al., 1980, supra. The narrow host specificity of S. typhi has resulted in the extensive use of S. enteriditis typhimurium infection of mice as a laboratory model of typhoid fever, Carter et al., 1984 J. Exp. Med. 139:1189. S. typhimurium infects a wider range of hosts, causing acute gastroenteritis in man and a disease similar to typhoid fever in the mouse and cow.
Salmonella infections are acquired by oral ingestion. The organisms, after traversing the stomach, replicate in the small bowel, Hornik et al., 1970, N. Eng. J. Med. 283:686. Salmonella are capable of invasion of the intestinal mucosal cells, and S. typhi can pass through this mucosal barrier and spread via the Peyer's patches to the lamina propria and regional lymph nodes. Colonization of the reticuloendothelial cells of the host then occurs after bacteremia. The ability of S. typhi to survive and replicate within the cells of the human reticuloendothelial system is essential to its pathogenesis, Hook et al., 1980, supra, Hornick et al., 1970, supra, and Carter et al., 1984, supra.
Immunity to Salmonella typhi involves humoral and cell-mediated immunity, Murphy et al., 1987, J. Infect. Dis. 156:1005, and is obtainable by vaccination, Edelman et al., 1986, Rev. Inf. Dis. 8:324. Recently, human field trials demonstrated significant protective efficacy against S. typhi infection after intramuscular vaccination with partially purified Vi antigen, Lanata et al., 1983, Lancet 2:441. Antibody-dependent enhancement of S. typhi killing by T cells has been demonstrated in individuals who received a live S. typhi vaccine, indicating that these antibodies may be necessary for the host to generate a cell-mediated immune response, Levine et al., 1987, J. Clin. Invest. 79:888. The cell-mediated immune response is important in typhoid immunity since killed vaccines that do not induce this immune response are not protective in man, Collins et al., 1972, Infect. Immun. 41:742.